Public lighting systems are one of the electrical loads consuming the highest level of energy, and therefore, any improvement to the efficiency of these systems results in a great saving of energy for the companies producing electrical energy, which results in less consumption of fossil fuel and less air pollution. One of the most efficient sources currently known is the sodium-vapor street light; its high efficiency is one of the main reasons why it is preferred in public lighting systems. The combination of a sodium-vapor street light along with an electronic ballast results in significant saving of electrical energy, and if a luminous intensity control reducing the energy consumption at dawn is added thereto, the result is a very important saving of energy compared to a conventional system using mercury-vapor street lights and electromagnetic ballast.
Some energy saving devices use a micro controller for the control of the electronic ballast elements which supply the sodium-vapor street light, allow greater flexibility for the control of the luminous intensity, the protections associated with the lighting process of the lamp, and also reduce the number of necessary components and the size of the ballast. Such devices also allow the implementation of techniques for simple elimination of acoustic resonances without adding additional elements.
Generally, sodium-vapor street lights require a lighting voltage above 2000 volts; in order to provide these voltage levels, an igniter is usually used. The use of this additional component increases the ballast cost, hence, the ballast resonator should be capable of providing these voltage levels. An alternative solution is to use a resonant tank circuit which provides the sufficient voltage to light a lamp. The resonant tank circuit is capable of providing high voltage levels during a very brief time. When the time is extended, the high currents involved in the lighting process may damage the semiconductors devices of the resonator. This condition may occur when no lamp was connected to the resonator or when a lamp has just turned off and it is intended to immediately re-light the lamp after turning it off. In order to avoid this damage, a protection is necessary which detects if the lamp has already turned on and otherwise deactivates the ballast.
Another risk condition of the energy saving devices occurs when the electronic ballast supply voltage is lower than the nominal voltage; according to the standards, all electronic ballast may include a correcting stage of the energy factor which provides a voltage level and continuous energy to the ballast resonator. If the supply voltage falls below a critical level, the current requested by the energy factor corrector is increased at the same ratio and it may damage the semiconductor devices of the corrector. Therefore, a protection deactivating the electronic ballast is necessary under low supply voltage conditions.
Ohkubo and Miyagaki proposed electronic ballast in U.S. Pat. No. 5,482,860, which includes a microprocessor that is mainly used to program a control method preventing the acoustic resonant phenomenon. The disadvantage of the control method described in this patent consists in that the protection sequence or the lighting process of the lamp are not established.
Electronic ballast for high intensity discharge lamps capable of providing high voltage levels for lighting of the lamp is presented in U.S. Pat. No. 5,677,602, the ballast includes a protection to detect the lighting of the lamp. However, this protection uses an operational amplifier for the detection of current which increases the final product cost.
U.S. Pat. No. 6,137,240 discloses a control circuit for a universal ballast based on a micro controller. The universal ballast may turn on, stabilize and control the luminous intensity of the lamp. The ballast may have a corrector of the energy factor based on an elevator converter and may establish options to supply the micro controller from the elevator converter. The above-mentioned control circuit presents the following disadvantages: programming of the micro controller does not include any action to eliminate the acoustic resonance phenomenon; it neither includes protection against absence of the lamp or protection of the elevator converter against supplying from Alternating Current (AC) sources of low voltage.
U.S. Pat. No. 6,329,761 discloses electronic ballast for high intensity discharge lamps allowing the control of luminous intensity and presents a high energy factor. However, this invention does not use a micro controller and for the lighting of the lamp it uses a special circuit for this function which increases the number of components and its complexity and, therefore, the cost.
Notwithstanding the above-mentioned description in the technical field, there is still the need of an energy saving device for public lighting systems which is easy, efficient, includes a luminous intensity control, low cost and exceeds the energy savings provided by known systems.